1. Field of the Invention
The present invention relates to a method for detecting the presence of Bloch lines in a magnetic wall of a striped magnetic domain formed in a thin magnetic layer, said Bloch line being used as a unit information carrier in a Bloch line memory of a very high density.
2. Related Background Art
Various memory devices, such as magnetic tape, the Winchester disk, floppy disk, optical disk, magneto-optical disk, magnetic bubble memory etc. are being used for external memories of computers, electronic files, image files or the like. All these memory devices, with the exception of magnetic bubble memory, involve a relative movement of a recording medium such as a tape or a disk and a record/reproducing head, and are therefore associated with the problems of tracking, gliding and abrasion between the recording medium and the head, and defocusing in the case of optical or magneto-optical disk, which hinders achievement of a high density in the memory.
On the other hand, the magnetic bubble memory, as disclosed in the U.S. patent application Ser. No. 434,538, which is a continuation of U.S. patent application Ser. No. 801,401, now abandoned, of the same assignee, not involving a mechanical driving mechanism and having a high reliability, has been considered more favorable for achieving a high density in comparison with other memories. However, such magnetic bubble memory utilizes, as the information carrier of 1 bit, a circular magnetic domain or a bubble formed in a magnetic garnet layer having an axis of easy magnetization perpendicular to the plane of said layer, and is therefore associated with a limit of several tens of megabits per chip even when there is employed the minimum bubble (0.3 .mu.m in diameter) limited simply by the properties of the presently available garnet layer. It is therefore not possible to achieve a high density in the magnetic bubble memory unless an alternative material such as hexaferrite or an amorphous alloy becomes available.
In order to overcome the above-mentioned limit in the recording density of the magnetic bubble memory, attention has recently been attracted to the Bloch line memory. The assignee of the present invention has already disclosed related technologies for this type of memory in U.S. patent application Ser No 660,260, which is a continuation of U.S. patent application Ser. No. 800,770, now abandoned, and U.S. Pat. No. 4,974,201, which issued on Nov. 27, 1990, from U.S. patent application Ser. No. 072,668, filed on July 28, 1987.
The Bloch line memory utilizes, as described in U.S. Pat. No. 4,583,200, in a magnetic wall surrounding a magnetic domain formed in a magnetic garnet film, an area composed of a Neil magnetic wall (hereinafter called a Bloch line, constituting an information carrier of one bit) sandwiched between Bloch magnetic wall structures in which the direction of magnetization in the magnetic wall is twisted into the opposite direction, and is capable of achieving a recording density higher by almost two orders than that in the magnetic bubble memory utilizing the circular magnetic domain, called a bubble, as one bit. For example a garnet film with a bubble diameter of 0.5 .mu.m can achieve a memory capacity of 1.6 Gbits per chip using the Block line.
FIG. 1 schematically illustrates a conventional Bloch line memory, wherein are shown a substrate 1 composed of a rare earth garnet such as GGG or NbGG; a magnetic garnet film 2 formed for example by liquid phase epitaxy (LPE) on said substrate 1; stripe-shaped magnetic domains 3; and conductor lines 4 patterned on the magnetic garnet film 2. A bias magnetic field H.sub.B is applied, as indicated by an arrow, to the entire memory. In the magnetic wall of the stripe-shaped magnetic domains 3, information is stored by the presence or absence of a pair of Bloch lines, respectively corresponding to "1" or "0". Each Bloch line pair is present at a stable point, or a potential well, formed in the stripe-shaped magnetic domain 3, and is transferred to an adjacent potential well by the application of a pulse magnetic field perpendicular to the plane of the substrate. In the following there will be explained the method of reading information from the Bloch line memory, or of detecting the presence of Bloch line pairs.
FIGS. 2A to 2C are schematic views showing the conventional Bloch line detecting method, wherein the same components as those in FIG. 1 are represented by the same numbers. Also there are shown a magnetic wall 5; a separated bubble 6; and Bloch lines 7. Arrows in the magnetic wall 5 indicate the direction of magnetization, and arrows in the conductor lines 4 indicate the direction of current.
Referring to FIG. 2A, a stripe-shaped magnetic domain 3 is formed on the magnetic garnet film 2, and a Bloch line 7 is present in the magnetic wall 5. However, the potential well is not illustrated. Across the magnetic domain 3 there are provided two conductor lines 4 in which mutually opposite pulse currents are given as indicated by the arrows. Since the magnetic field formed by the currents in the conductor lines 4 is opposite to the direction of magnetization in the stripe-shaped magnetic domain 3, the magnetic domain positioned between the conductor lines 4 shrinks, to cause a displacement of the magnetic walls 5, as indicated by broken lines. When the currents are further increased, the magnetic walls 5 are mutually united as shown in FIG. 2B whereby the end portion of the magnetic domain 3 is separated as a bubble 6. After the currents are terminated, a Bloch line 7 the same as that present before the separation of the bubble is formed at the end portion of the magnetic domain 3, and said magnetic domain 3 recovers the original size. FIG. 2C shows a case where the Block line is absent. In this state, currents supplied to the conductor lines 4 allow one to move the magnetic walls positioned between the conductor lines 4 in a similar manner as in the presence of Bloch line shown in FIGS. 2A and 2B and said magnetic walls can be mutually united by an increase in the currents. However, in case of FIG. 2A, with the presence of a Bloch line, the directions of magnetization in the magnetic walls positioned between two conductor lines are the same, while in case of FIG. 2C said directions are mutually opposite, so that the interaction of the magnetic walls at said uniting is different in these two cases. More specifically, the current required for uniting the magnetic walls is smaller in the case Bloch line. It is therefore possible to obtain a separated bubble, corresponding to the presence of a Bloch line, by selecting the currents in the conductor lines 4 at such level between a value required for uniting the magnetic walls 5 in the presence of the Bloch line 7 and a value required for uniting said magnetic walls in the absence of the Bloch line, and to detect the presence of a Bloch line 7 by detecting the bubble 6 in the same manner as in the conventional magnetic bubble memory.
However, the conventional method of Bloch line detection explained above requires the separation of the stripe-shaped magnetic domain for each Bloch line detection, requires a complex mechanism, and cannot achieve a high detection speed, because the separated bubble has to be transferred and detected with an external rotating magnetic field or a current drive method. Also, this method requires a high electric power consumption.